The two-pion production in nucleon-nucleon collisions has been studied by exclusive and kinematically complete experiments from threshold up to T(p) = 1.36 GeV at CELSIUS-WASA. At near-threshold energies the total and differential distributions for the pi(+)pi(-) and pi(0)pi(0) channels are dominated by Roper excitation and its decay into N sigma and Delta pi channels. At beam energies T(p) > 1.1 GeV the Delta Delta excitation governs the two-pion production process. In the pi(+)pi(+) channel evidence is found for the excitation of a higher-lying I=3/2 resonance, favorably the Delta(1600). The isovector fusion processes leading to the deuteron and to quasi-stable (2)He, respectively, exhibit no or only a modest ABC-effect, i.e. low-mass enhancement in the pi pi-invariant mass spectrum, and can be described by conventional t-channel Delta Delta excitation. On the other hand, the isoscalar fusion process to the deuteron exhibits a dramatic ABC-effect correlated with a narrow resonance-like energy dependence in the total cross section with a width of only 50 MeV and situated at a mass 90 MeV below the Delta Delta mass.